2023 CSAT, all questions

UPSC Prelims 2023 · CSAT

Question 29: Basic Numeracy

What is the sum of all digits which appear in all the integers from 10 to 100 ?

a855
b856
c910
d911

AnswerUPSC’s answer is (b).

Tens digits of 10 to 99: each digit 1 to 9 appears 10 times, so 10 x 45 = 450. Units digits of 10 to 99: each digit 0 to 9 appears in 9 numbers, so 9 x 45 = 405. The number 100 adds 1 + 0 + 0 = 1. Total = 450 + 405 + 1 = 856.

Why each option is right or wrong
  • (a) Wrong: leaves out the 1 from 100.
  • (b) Right: 450 + 405 + 1 = 856.
  • (c) Wrong: overcounts.
  • (d) Wrong: overcounts.

Answer: UPSC’s key. Explanation: written by UPSC Grade, not by UPSC.

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Source: UPSC, Civil Services (Preliminary) Examination 2023, General Studies Paper II (CSAT), Series A. The paper on upsc.gov.in.